Suppose a store sells ground beef for $5.00 per pound. At this price, they sell an average of 250 pounds of ground beef each day. When the store puts ground beef on sale for only $3.00 per pound, they sell 400 pounds of ground beef that day. Assuming a linear relationship between the price per pound of ground beef and the demand (in pounds), find a model for the d, the daily demand of ground beef as a function of p, the price in dollars for one pound.